Tunnel on Raspian sometimes not responding to pings

[Spodi]

Hello Community,

i don't know if this is a bug on my Raspian installation or a specific thing I missed on the tunnel specifications.

For some reason I sometimes can't ping my tunnel ip, wich is located on my raspberry. It also doesn't accept SSH connections to its ipv6 adress when this happens. Clients using the raspberry as ipv6 router to use its ipv6 interface are working fine. Pinging another client or ifdown and ifup at the tunnel interface seems to fix the problem for a short time, wich is really weird.

Maybe someone knows what can cause this?